1. Field of the Invention
This invention relates to new liquid crystal substances which exhibit a liquid crystal phase up to high temperatures and have a positive dielectric anistoropy, and liquid crystal compositions containing the same.
2. Description of the Prior Art
Display elements using liquid crystals have come to be widely used for watches, desk calculators, etc. Such liquid crystal display elements utilize optical anisotropy and dielectric anisotropy of liquid crystal substances, and their liquid crystal phases include nematic liquid crystal phase, smectic liquid crystal phase and cholesteric liquid crystal phase, and those utilizing nematic liquid crystals among them have been most broadly employed for practical uses. Further, liquid crystal display elements include those of TN type (twisted nematic type), DS type (dynamic scattering type), guest-host type, DAP type, etc. and properties required for liquid crystal substances used therefor are varied. At any rate, however, as liquid crystal substances used for these display elements, those which exhibit a liquid crystal phase within as broad a range as possible in the neutral world have been desired. However, no single compounds satisfying such a requirement have been found to date, and it is the present status that liquid crystal compositions which are practically usable for the present have been obtained by mixing together several kinds of liquid crystal compounds or non-liquid crystal compounds. Further, these substances, of course, must be stable to moisture, heat, air, etc., and moreover, it has been desired for the substances that the threshold voltage and saturation voltage required for driving the display elements be as low as possible and the viscosity be as low as possible for making the response speed higher. As a compound which satisfies such requirements to a certain extent, a compound expressed by the formula ##STR2## (m.p.: 62.degree. C., c.p. (clearing point): 85.degree. C.) is disclosed in U.S. Pat. No. 4,181,625. However, the recent technical advance of liquid crystal display elements is remarkable, and liquid crystal cells operated within a broad temperature range between still lower ones and still higher ones have been required. Further, for broadening the liquid crystal temperature range toward the higher temperatures, it is necessary to employ a high melting liquid crystal substance as a component, but such a high melting liquid crystal substance generally has a high viscosity and accordingly a liquid crystal composition containing it also has a high viscosity; hence there has been a tendency that the response speed of liquid crystal display elements as usable up to e.g. 80.degree. C., particularly that at lower temperatures, becomes notably slow.
The present inventors have made various studies on liquid crystal compounds meeting the above requirements and as a result have found liquid crystal substances having a high nematic-clearing point (hereinafter abbreviated to N-I point) and a broad nematic temperature range and yet a low viscosity.